The present invention concerns a catalyst, adapted for connection into an exhaust gas line, for the catalytic purification of the exhaust gases of an internal combustion engine. More particularly, the invention concerns a catalytic unit having a monolithic catalyst carrier body provided with a suitable catalytic agent. The carrier body is supported in a housing by means of an intermediate layer of material arranged in compression between the circumferential surface of the carrier body and the housing.
The U.S. patent application Ser. No. 403,270, filed Oct. 3, 1973 by Joachim Neumann (which is commonly assigned with the present application) discloses a method of producing a catalytic unit in which the circumferential surface of a monolithic catalyst carrier body is covered by an intermediate layer of elastic, heat-resistant material, and the package, so formed, is supported in a housing under sufficient radial stress to prevent axial movement of the carrier body during normal operation. The intermediate layer preferably consists of one or more plies of a mat made of a ceramic fiber material; specifically, an alluminum-silicate-fiber material.
In this manner, the catalyst carrier body, which consists of a delicate porous ceramic substance, is safely mounted in a housing and can withstand the high mechanical and thermal stresses which prevail, for example, when the catalytic unit is mounted in an automobile for exhaust gas purification. In addition, arrangement for supporting the carrier body is simple and economical in design.
During operation of catalytic exhaust gas purification units of the type provided with a monolithic carrier body (such body normally consisting of a cylindrical ceramic body having a plurality of continuous channels extending through it in the longitudinal direction), it has been found that cracks or fissures frequently appear in the ceramic material. These fissures, when coupled with the high mechanical stresses, caused especially by vibrations originating with internal combustion engine of an automobile, result in the destruction of the ceramic body after a comparatively short operating time and thereby render the exhaust gas purification unit inoperative.
Longitudinal fissures in the monolithic carrier body have proven to be particularly problematic. The principal cause of these fissures has been found to be the outer tangential tensile stresses produced by thermal action, expecially during engine warm-up periods, which is nonuniform across the monolith cross-section.